Electrostatic transfer method

ABSTRACT

An improved method of successively transferring a plurality of powder images, such as toner images of different colors, from an electrostatic plate onto a single copy sheet by employing an electrically biased transfer drum to bring the copy sheet in contact with the electrostatic plate during successive transfer steps. During transfer of the first toner image, the transfer drum is biased at a first potential and during the transfer of all succeeding toner images, the transfer drum is biased at a potential greater than the first potential, the potential applied to the transfer drum during any one succeeding transfer step being of greater value than the potential applied thereto during the immediately preceding transfer step.

United States Patent 1191 Langdon [45] Apr. 24, 1973 ELECTROSTATIC TRANSFER 3,363,555 1/1968 Olden ..96/1.4 x

METHOD OTHER PUBLICATIONS [75] Inventor: Michael J. Langdon, Penfield, N.Y.

Andrus et al., Principles of Image Transfer and Fixa- [73] Asslgnee: Xemx corporatmn Stamford tion Xerography and Related Processes, Focal Press Com (1965) pp. 391398. [22] Filed: July 15, 1971 Primary Examiner-Charles E. Van Horn 1211 Appl' 162397 Azt0mey-James J. Ralabate et 31.

Related US. Application Data [57] ABSTRACT [63] Continuation-impart of Ser. No. 830,426, June 4,

1969Yabandone An improved method of successwely transferrmg a plurality of powder images, such as toner images of 52 US. Cl. ..96/l.2, 96/14, 117/175, different colors, from an electrostatic plate Onto a Int. Cl.....G03g 13/22, G03g 13/16, G03g 15/22 gle copy sheet by employing an electrically biased transfer drum to bring the copy sheet in contact with the electrostatic plate during successive transfer steps. During transfer of the first toner image, the transfer drum is biased at a first potential and during the transfer of all succeeding toner images, the transfer drum is biased at a potential greater than the first potential, the potential applied to the transfer drum during any one succeeding transfer step being of greater value than the potential applied thereto during the immediately preceding transfer step.

8 Claims, 10 Drawing Figures Patented April 24, 1973 6 Sheets-Sheet l I INVENTOR. MICHAEL J. LANGDON l ll ATTORNEY Patented April 24, 1973 6 Sheets-$heet 2 Patented April 24, 1973 3,729,311

6 Sheets-Sheet 5 Patented April 24, 1973 VOLTAGE CONTROL CIRCUIT 6 Sheets-Sheet 4 RELAY CONTROL D. C. POWER SOURCE TRANSFER SWITCH POWER SOURCE ELECTRICAL BRUSH Patented April 24, 1973 6 Sheets-Sheet 6 ELECTROSTATIC TRANSFER METHOD This is a continuation-in-part of application Ser. No. 830,426 filed June 4, 1969, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method of transferring toner images from an electrostatic plate to a copy sheet and, more particularly, to the successive transfer of powder images onto the same copy sheet.

In a copying system such as those which employ transfer or powder imaging, a uniform electrostatic charge is generated on the surface of a photoconductive plate and the plate is exposed to a light image conforming to the information to be copied. A latent electrostatic image is created which is then developed with a finely divided powder material, referred to herein as toner. The toner image thus created is transferred from the surface of the plate to a copy sheet thereby forming a copy ofthe information being reproduced.

One technique used to transfer the toner image from the surface of the plate to the copy sheet is to place the copy sheet between the plate and a transfer drum having a conductive core and a relatively non-conductive surface material and apply an electrical potential to the core of the transfer drum as it rotates to bring the copy sheet in contact with the plate. The application of potential to the transfer drum forms a field between the transfer drum and plate which causes the toner image to be attracted to the copy sheet. As a result of this attraction, the toner remains on the copy sheet when the latter is removed from the plate. This transfer technique works well to transfer toner images onto clean copy sheets over a range of voltages, however, when it is desirable to transfer more than one toner image to the same copy sheet in successive transfer steps, the transfer efficiency decreases as toner builds up on the copy sheet. As a result of this decrease in transfer efficiency, only portions of toner images are transferred to the copy sheet in succeeding transfer steps while the remainder of the toner images remain on the electrostatic plate. It is believed that transfer efficiency decreases as toner builds up on the copy sheet because the layer of toner already of the sheet decreases the attractive force on the transfer drum during succeeding transfer steps.

In copying systems in which a plurality of different color toner images are transferred to a single copy sheet in successive transfer steps, this decrease in transfer efficiency described above becomes critical. For instance, multicolor copies can be made by forming toner images of different colors and successively transferring the toner images onto the copy sheet on top of one another to form a multicolor copy composed ofthe different color images. lftransfer efficiency is allowed to drop off in the manner described above as successive toner images of different colors are transferred to the copy sheet, the colors which are transferred first will become dominant and will cause an unbalance of color in the final multicolor image on the copy sheet. The method of transferring a plurality of toner images to a copy sheet described herein eliminates the problem of decreasing transfer efficiency as the toner builds up on the copy sheet. In multicolor copying systems in which a plurality of color toner images are transferred to a single copy sheet to form a composite multicolor copy, the employment of the present transfer method enables each color image to be transferred with approximately the same transfer efficiency thereby assuring that the final toner image on the copy sheet maintains good color balance.

The method of transfer disclosed herein applies a successively increasing electrical potential to the conductive core of a transfer drum as successive transfer steps are carried out. The effect of the increased potential is to offset the decrease of the attractive force on a toner image being transferred due to the presence of toner already on the copy sheet. As a result, the transfer efficiency of each successive transfer step remains substantially constant regardless of the amount of toner already on the copy sheet.

Accordingly, it is a further object of the invention to improve the transfer of a plurality of toner images from an electrostatic plate to the same copy sheet.

It is a further object of the invention to improve the transfer of plurality of toner images of different colors from electrostatic plate to the same copy sheet.

It is a further object of the invention to improve the transfer of toner images from an electrostatic plate onto a copy sheet over a toner image already n the copy sheet.

It is a further object of the invention to maintain the same transfer efficiency during successive transfer steps in a multi-transfer process in which a plurality of toner images are transferred to the same copy sheet on top of one another.

SUMMARY OF THE INVENTION The present invention is a method of transferring a plurality of toner images onto the same copy sheet. A copy sheet is brought into contact with each toner image after it is formed on an electrostatic plate by a transfer drum having a conductive core and a relatively non-conductive material on its surface. The electrical potential applied to the core of the transfer drum during each successive transfer step is increased over that of the immediately preceding transfer step so that substantially the same attractive force attracts each toner image throughout the multiple transfer process thereby assuring good efficiency in all transfer steps.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description to be used in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic illustration of the invention in a color copying machine.

FIG. 2 is a cut-away view of the transfer drum.

FIG. 3 is a partial section view of FIG. 2 taken through section 33.

FIG. 4 is a partial section view of FIG. 2 taken through section 3-3.

FIG. 5 is a partial section view of FIG. through section 4-4.

FIG. 6 is a view of the cam and follower which together control the operation of the register stops and gripper fingers on the transfer drum.

FIG. 7 is a rear view of the transfer drum and sup; porting mechanisms.

FIG. 8 is a detailed view of the mechanism which controls the position of the cam shown in FIG. 6.

drum and sup- DESCRIPTION OF THE PREFERRED EMBODIMENT The apparatus described herein is an electrostatic reproduction system which reproduces a multicolor copy from a multicolor original such as a document. Referring to FIG. 1 there is shown a color copying machine which reproduces an original in color xerographically. The apparatus for reproducing multicolor images includes an electrostatic drum or photoreceptor 90, the surface of which can include a photoconductive material overlying a conductive material, which rotates through various xerographic processing stations; charging station 10, exposing station 20, developing station 30, transfer station 40, and cleaning station 50. The photoreceptor rotates with shaft 91 in the direction indicated by the arrow through the stations mentioned above in a processing cycle, the approximate positions of the various stations being shown by the brackets next to the drum surface in FIG. 1.

The photoreceptor makes a plurality of revolutions; for instance, two or three revolutions, through the processing stations in order to carry out a multicolor copying cycle. During each revolution, a latent electrostatic image corresponding to one of the colors in the original is formed on the surface on the photoreceptor and developed with a finely divided, pigmented materi al such as toner of the corresponding color, and, then, the toner image is transferred from the photoreceptor to a copy sheet at the transfer station. The toner images, each of a different color, that are formed on the photoreceptor in each revolution of a copying cycle are transferred to the copy sheet in registration with one another and the composite toner image resulting on the copy sheet after the copying cycle has been completed is a multicolor copy of the original.

In the machine shown in FIG. 1, the developing station contains three separate developing assemblies31, 32, and 33. Although each developing assembly is mechanically similar, the color of the toner applied to the surface of the photoreceptor 90 by each developing assembly is different. In the apparatus shown, for example, the toner colors in the developing assemblies 31, 32, and 33 are yellow, cyan, and magenta, respectively. The three toner colors can be developed in any convenient color order and the different color images formed can be placed on the copy sheet in any order. The developing assemblies are selectively operated during a copying cycle so that only one of the assemblies applies toner to the surface of the photoreceptor during each revolution. Thus, in the system shown in FIG. 1, during the first revolution yellow toner is applied to the surface of the photoreceptor by developing assembly 31 while developing assemblies 32 and 33 remain in an inoperative condition. Then, during the second revolution of the photoreceptor, cyan toner is applied to the surface of the photoreceptor by developing assembly 32 while developing assemblies 31 and 33 remain in an inoperative condition. Finally, during the third revolution of the photoreceptor, magenta toner is applied to the surface of the photoreceptor by developing assembly 33 while developing assemblies 31 and 32 remain inoperative. In this manner, toner images of each of the three developing colors used in the apparatus shown in FIG. 1, yellow, cyan, and magenta, are formed on the photoreceptor and then transferred to the copy sheet during successive revolutions of the photoreceptor.

A latent electrostatic image is formed on the surface of the photoreceptor during each revolution by first placing a uniform charge on its surface and then exposing thecharged surface to a light image corresponding to the particular color toner being applied to the photoreceptor by a developing assembly during that revolution. Any suitable device 21 can be used to form the light images. Similarly, any suitable charging means can be utilized at station 10 to charge the surface of the photoreceptor such as the corona charging device indicated by reference number 11. Exposing station 20, in addition to having means to expose the photoreceptor to light images as described above, can include an inter-image erasing device which dissipates, or erases, the charge on the surface of the photoreceptor between latent images. This apparatus can be any suitable device for dissipating the charge on the photoreceptor such as an electroluminescent panel 22 which is activated only between latent images. The inter-image erasing device is a desirable feature when a latent image formed on the photoreceptor does not completely cover the periphery of the photoreceptor since these areas would otherwise completely develop out as the photoreceptor passes through the developing station and cause a waste of toner.

The developing station 30 includes three identical developing apparatus 31, 32, and 33 which apply toner particles to the latent image on the photoreceptor surface. These three developing assemblies, all of which are normally in an inoperative condition, are brought into an operative condition selectively in accordance with the color toner to be placed on the photoreceptor during any particular revolution. The housing of developing assembly 32 is broken away so that the internal elements of the assembly can be seen. The members which apply toner to the photoreceptor are magnetic brushes 35 which bring magnetic developer, a mixture of magnetic carrier particles and toner particles, into contact with the surface of the photoreceptor 90. The developing assembly is contained within housing 39 and is replenished with toner particles from supply container 34 as the toner is used. The developer is moved to the upper portion of the housing, which contains magnetic transporter 37 and movable gate 38, by agitator 36. Upon reaching the upper portion of the housing the developer is attracted to the magnetic transporter which rotates in the clockwise direction to convey the developer toward the vicinity of gate 38.

Depending upon the position of gate 38, the developer is either passed onto magnetic brushes 35 to be applied to the photoreceptor or is dropped from the transporter directly into the lower portion of the housing without touching the magnetic brushes. The developing assembly is in an inoperative condition when the gate 38 is in the position shown in the solid lines. In this position the gate acts as a guide to direct the developer onto the magnetic transporter away from the magnetic brushes and down directly into the lower portion of the housingHWhen the gate is in the position shown in dotted lines the developing assembly is placed in the operative condition. In the inoperative position the gate acts as a scrapper and guide which frees the developer from the magnetic transporter 37 and directs it onto the magnetic brushes 35. The developer is brought into contact with the surface of the photoreceptor by the upper magnetic brush where it is again brought into Contact with the surface of the photoreceptor. The toner particles in the developer are attracted from the carrier particles to the photoreceptor as the developer is placed adjacent the photoreceptor by the magnetic brushes thereby developing the latent image thereon.

Each developing assembly operates in the manner described above, the latent image on the photoreceptor being developed by that particular developing assembly which has its gate in the position shown in dotted lines. Due to the simplicity of the gate device described above, each developing assembly can be maintained in a standby condition since its agitator, magnetic transporter and magnetic brushes can continue to rotate even through the assembly is not applying toner particles to the photoreceptor.

After the toner image is formed on the surface of the photoreceptor it is transferred from the photoreceptor to a copy sheet in the transfer station 40. Transfer drum 42 is adapted to convey a copy sheet through the transfer station in contact with and in registration with the toner image on the photoreceptor. ln sheet feeding apparatus 70, an individual copy sheet is fed to the transfer drum as needed from a stack of sheets 72 by feed roller 71 which moves the sheets through guides 73 and onto the surface of the transfer drum. The copy sheet is fastened to the transfer drum 42 by a series of grippers and the drum carries the copy sheet in three revolutions through the transfer station to transfer the plurality of color images to the copy sheet. The transfer drum has the same size circumference as the photoreceptor and both rotate at the same speed, therefore, once the copy sheet is aligned with the grippers on the drum it is also in registration with the photoreceptor during all three transfer steps. The transfer station 40 includes pretransfer corona charging device 41 which adjusts the electrostatic charge on the toner particles to prepare the toner image from transfer to the copy sheet. The transfer drum 42 has a conductive core with a layer of relatively non-conductive material on its periphery. An electrical bias is applied to the conductive core of the transfer drum during the transfer step to create an electrostatic field between the photoreceptor and a copy sheet which urges the toner image from the photoreceptor to the copy sheet.

After making a plurality of revolutions on the transfer drum the copy sheet is stripped from the surface of the transfer drum by fingers 64 and conveyed into a fusing apparatus 60 by belt conveyors 62 and 63 where the fusing housing 61 fixes the toner image to the copy sheet. After the toner image is fixed to the copy sheet, the copy sheet is guided into vacuum transport assembly 80 by guides 65. The vacuum transport assembly includes a plurality of belts 83 and holes 82 to impose a vacuum between the belts. The vacuum and belt arrangement carries the copy sheet from the area of guides 65 towards storage tray 84 where it is stored.

After each toner image is transferred to the copy sheet, the surface of the photoreceptor is cleaned in preparation for subsequent revolutions in station 50. Cleaning station 50 includes a precleaning corona charging device 51 and a brush cleaning device 52 which act together to remove any residue toner remaining on the surface of the photoreceptor after transfer has taken place. Any toner that is removed by brush 52 is withdrawn from the brush into a filter bag apparatus 53 where it is held separate from the reproduction system.

Referring to FIG. 2, the transfer drum has a hollow, conductive core 102 with a layer 104 of insulating material. The cylindrical core 102 is made of any suitable conductive material such as aluminum and has a thin wall in order that the drum is lightweight and that the various mechanisms inside the drum have adequate room to function. The end plates 103 and 105, which are supported by and turn with main transfer drum shaft 101, are made of any suitable insulating material such as insulating plastic which effectively insulates the conductive core 102 from other parts of the machine. The outside surface of the conductive core contains a layer 104 of any suitable relatively non-conductive material, such as a rubber-like insulating material, upon which the copy sheet is supported as it is brought through the transfer station. The layer 104 is preferably made of a rubber-like material so that it yields and, consequently, is not likely to mar the photoreceptor if it should be brought into contact with it.

A ring 116, made of a conductive material such as brass, is fastened to end plate 105 in a concentric manner with shaft 101 and functions to transmit an electrical bias from stationary brush 117 to the conductive core of the drum. The brass ring 116 is secured to the end plate 105 by a plurality electrically conducting, metal screws 118. The metal screws pass through the end plate and are threaded into support ring 119 which, in turn, is in contact with the conductive core 102. The electrical bias on brush 117 passes through the ring 116, the screws 118, and support ring 119 to conductive core 102. The other end plate of the transfer drum end plate 103, also has a support ring 120 and is fastened thereto by screws 125.

The transfer drum contains three sets of registration stops 113 and gripper fingers 112 which are keyed to shaft 111 and which operate to register and grip the leading edge of the copy sheet before the drum takes it through the transfer station. Shaft 111 and frame 124 are mounted on the support plates and 132, shaft 111 being able to rotate about its longitudinal axis in response to the movement of the follower arm 106. Arm 123, together with a spring 115 (shown in FIG. 3) which biases the shaft 111 in one direction, arms 126 which control the movement of register stops 113, and arms 131 which support gripper fingers 112 are supported by and turn with support shaft 111.

The transfer drum also contains frame 124 which is fastened to support plates 130 and 132. In addition to supporting bracket 122, to which the spring on the end of arm 133 is fastened (shown in FIG. 3), the frame 124 contains slots 129 which guide the up and down movement of register stops 113. The movement of shaft 111 is controlled by a stationary cam 201 (shown in FIG. 6) in conjunction with follower 107 which rotates with the transfer drum. Follower 107, as it rotates with the transfer drum, moves about the periphery of fixed cam 201. The movement of the followers, in turn, causes arm 106 to move, and, since arm 106 is supported by and keyed to shaft 111, the shaft 111 rotates in response to the movement of the arm 106.

Referring to FIG. 3, which is a partial view of the transfer drum in FIG. 2, taken through section 3-3, spring 115, acting through arm 123, biases shaft 111 in the counterclockwise direction. Internal frame 124 has spring support 122 attached to it which is suspended towards the center of the transfer drum and away from the free end of arm 123. The spring 115 is attached between the free end of arm 123 and the tip of the spring support 122. Since arm 123 is keyed to shaft 111, the spring 115 acts to bias the shaft in the counterclockwise direction thereby biasing the register stops 113 in their furthest downward position (see FIG. 4) and the gripper fingers 112 in their most clockwise direction (see FIG. 5).

Referring to FIG. 4, which is a partial view of the transfer drum shown in FIG. 2 taken through section 44, register stops 113 are guided in a straight line movement in a plane which is substantially normal to the surface of the transfer drum by the slot 140 in the transfer drum and the slot 129 in the frame 124. Arm 126, which is keyed to shaft 111, turns with the shaft to move register stops 113 either up or down depending on the direction of rotation of shaft 111. Slot 127, which is positioned in the end portion of arm 126, is adapted to enable pin 128, which is fastened to register stop 113, to slide within it thereby transferring the rotary motion of the arm 126 to the linear motion of the register stop. As shaft 111 rotates in the counterclockwise direction, arm 126 rotates in the counterclockwise direction, thereby driving register stop 113 in a linear path towards the center of the transfer drum. Then, when shaft 111 rotates in the clockwise direction arm 126 rotates in the clockwise direction thereby driving register stop 113 away from the center of the transfer drum to bring it above the surface of the transfer drum. In FIG. 4, the register stops 113 are shown in their furthest downward position.

Referring to FIG. 5, which is a partial view of the transfer drum shown in FIG. 2 taken through section 55, the gripper fingers 112 are supported by arm 131 which, in turn, is keyed to shaft 111. As shaft 111 turns in the counterclockwise direction, gripper fingers 112 rotate in the counterclockwise direction to bring the angled tip of the gripper finger 112 down and into contact with the surface of the transfer drum. Then, when the shaft 111 rotates in the clockwise direction arm 131 and gripper fingers 112 rotate in the clockwise direction to bring the tip of the gripper finger 112 above and away from the surface of the transfer drum. The tip portion ofthe gripper fingers 112 move through slots 145 in the transfer drum when shaft 131 turns in either direction. In FIG. 5, the gripper fingers 112 are shown in their extreme counterclockwise position.

The movement of shaft 111 regulates the operation of the gripper fingers and register stops in unison to register, grip and, then, release the leading edge ofa copy sheet. In operation, the shaft 111 first rotates in the clockwise direction from its biased position to bring the register stops and gripper fingers just above the surface of the transfer drum. A copy sheet is then fed onto the surface of the transfer drum until its leading edge is in registration with the register stops. When the leading edge of the copy sheet has been properly registered, shaft 111 rotates in the counterclockwise direction to bring the register stops below the surface of the transfer drum and the gripper fingers into the position where they press the leading edge of the copy sheet against the surface of the transfer drum. The transfer drum then continues to rotate through its plurality of consecutive revolutions during which the plurality of toner images are transferred from the photoreceptor to the copy sheet.

After the plurality of toner images have been transferred to the copy sheet and the copy sheet is to be removed from the transfer drum, shaft 111 rotates again in the clockwise direction, but to a greater extent than during the registering and gripping step. As a result the gripper fingers release the copy sheet and the register stops push the leading edge of the copy away from the transfer drum surface to such an extent that the leading edge also clears the tip of the gripper fingers. (This is possible since the register stops are joined to arms 126 at a greater distance from shaft 111 than the gripper fingers are joined from the shaft At this time stripper fingers 64 (shown in FIG. 1) are brought near the surface of the transfer drum and as the transfer drum continues to rotate, the copy sheet is completely separated from the drum surface and conveyed into the fuser 60 (also shown in FIG. 1 The register stops and gripper fingers then rotate counterclockwise with shaft 111 to return to the positions where they are biased by spring 115.

Referring to FIG. 1, transfer drum 42 has misfeed detector 43 adjacent it between sheet feeder 73 and the transfer station 40. The purpose of the misfeed detector is to detect when a copy sheet is improperly registered with the register stops, is not gripped properly by the gripper fingers, or has not been fed into the transfer drum at all. The detector is represented merely by a box adjacent the transfer drum in FIG. 1 since any suitable detector device can be used. For instance, a series of photocells can be placed adjacent the surface of the transfer drum with associated light sources, and, depending on where the leading edge of the sheet is positioned relative to the-stops or where the gripper finger is located relative to the copy sheet, i.e., over or under the sheet, or whether or not a copy sheet is on the transfer drum at all, certain photocells are activated. The various combinations of photocell signals can then be read by a logic circuit to detect if a copy sheet is on the transfer drum and is properly aligned. If a copy sheet is properly aligned on the transfer drum, it is allowed to pass through the transfer station where a plurality of toner images are transferred to it. However, if the logic circuit determines that the copy sheet is not properly aligned, it generates a signal which is directed to a solenoid 320 (see FIG. 7) which activates the transfer drum withdrawal apparatus. The withdrawal apparatus rotates the transfer drum away from the photoreceptor thereby preventing the toner image from being transferred to a misaligned copy sheet or to the transfer drum surface when no sheet is on the drum. The apparatus which enables the transfer drum to rotate away from the photoreceptor is described in detail below.

Referring to FIG. 2, the rotation of shaft 111 is controlled by cam 201 and follower 107. The shaft 111, arm 106, and follower 107 rotate with the transfer drum while cam 201 remains in the stationary position. As a result of this arrangement the follower travels around the periphery of the cam and rotates arm 106 in response to the peripheral shape of the cam. The rela tionship between the cam and follower can be seen best in FIG. 6. Transfer drum 42, along with shaft 111, arm 106 and follower 107, is driven in the clockwise direction by shaft 101, as shown by the arrow. Cam 201, on the other hand, remains stationary relative to shaft 101 and, when engaged with the follower 107, forces the follower to move away from and towards the center of the transfer drum by cam portions 155 and 160. Through arm 106, cam portion 155 causes the registration stops and gripper fingers to register and grip the leading edge of the copy sheet while cam portion 160 causes the register stops and gripper fingers to release the leading edge of the sheet and drive it away from the surface ofthe transfer drum.

Referring to FIG. 7, cam 201 has a hub 312 which is adapted to slide along shaft 101 so that it can move between an operative position, ,where it is engaged by follower 107, and an inoperative position, where it does not engage the follower. In this figure the cam and hub are shown in the operative position which places the hub against the end plate of the transfer drum 42. In its operative position, the cam 201 engages the follower and operates the registration stops and gripper fingers as described above. When it is desirable to move the cam out of engagement with the follower, for instance, when the copy is to be brought continuously through a plurality of revolutions on the transfer drum to effect transfer of a plurality of toner images to it, the cam is moved to the left thereby bringing it to its inoperative position.

Shaft 101, the shaft that supports and drives transfer drum 42, is mounted for rotation in frame members 304 and 308. Gear 351 and pulley 352 rotate on shaft assembly 350 which, in turn, is supported for rotation by frame 354. Frame 308 is adapted to rotate about shaft assembly 350 so that when the transfer drum 42 is moved away from photoreceptor 90, gears 351 and 353 remain in a meshed relationship to one another. A belt (not shown) driven by a suitable drive means rotates pulley 352 and gear 351 on shaft assembly 350. The gear 351 drives gear 353 which, in turn, drives the transfer drum 42 through shaft 101.

The movement of cam between its inoperative position and its operative position, which is shown in FIG. 7, is controlled by the position of arm 203. When a signal is received by solenoid 211 to move the cam 201 to the right to its operative position, pin 206 is moved by the solenoid to the left. Arm 203, a second arm (not visible) is on the other side of shaft 101, pivots about pin 204 thereby causing the tip 205 of the arm to force the cam 201 and hub 312 into its operative position against the action of spring 202. When the cam is in the operative position and a signal is received by the solenoid 211 to move it onto the inoperative position, pin 206 is allowed to move to the right causing arm 203 to rotate clockwise about pin 204. Due to the action of spring 202, (a second spring, not visible, is on the other side of shaft 101), which is fastened to hub 312, the cam 201 moves to the left and out of engagement with the follower.

FIG. 8 shows the view of the transfer drum in which part of the frame 306 is broken away from the mechanism which moves the cam so that the various elements of the mechanism can be clearly seen. Solenoid 211 moves pin 206 through solenoid arm 210, the arm moving either to the right and left depending on the signal received by the solenoid. Pin 204 is supported for rotation by an appendage of arm 304 and spring 202 is fastened to arm 304 to maintain a mechanical bias on the cam toward the inoperative position. Referring to FIG. 9, stripper fingers 64 on the top of the transfer drum when activated, are placed near the drum surface to help strip a copy sheet from the transfer drum after all the images have been transferred to the copy sheet. Referring to FIG. 7, stripper fingers 64 are supported for rotation by shaft 360 which, in turn, is supported for rotation in frames 306 and 308. The stripper fingers are activated by solenoid 211 also. Referring to FIG. 8, arm 275, which is keyed to shaft 360, rotates the shaft 360 to bring the stripper fingers in and out of position adjacent the surface of the transfer drum. Arm 220, which is supported to pivot on frame bracket 209, is in the shape of an L, the lower extremity of the arm having a slot 376 in which pin 206 travels. As solenoid arm 210 moves to the left bringing pin 206 into the clockwise or downward direction, the stripper fingers are brought adjacent the transfer drum surface. The linkage (not shown) between arm 275 and the horizontal extremity of arm 220 is similar to the linkage between pin 206 and slot 276, the arm 275 having a pin at its end which slides in a slot in the end of the horizontal extremity of arm 220. Through this linkage, arm 275 is forced down by arm 220 thereby rotating shaft 360 to bring the stripper fingers adjacent the surface of the transfer drum. Then, when the signal to solenoid arm 210 ceases, spring 202 causes arm 210 to move pin 206 to the right, the associated action of arms 220 and 275 and shaft 360 driving the stripper fingers away from the surface of the transfer drum. As a result of the linkage described above, the stripper fingers can come into contact with the transfer drum only during the time that cam 201 is in its operative position.

During the rotation of shaft 101, it is necessary to assure that cam 201 does not turn with the shaft, especially when the cam is in its operative position. Keeper arm 313 is intended for this purpose. The lower position of the keeper are 313 is fastened to hub 312 while its upper portion has a slot 314 which is supported by pin 315. As the hub 312 moves along shaft 101, the keeper arm 313 moves with it, the upper portion of the keeper arm always sliding on pin 315 which is supported by arm 304. Due to this slot and pin arrangement, cam 201 always remains in a stationary position relative to the follower arm 106 even though the shaft 101 and the transfer drum 42 are continuously rotating.

Referring to FIG. 9, transfer drum 42 is brought away from the photoreceptor when a copy sheet is misfed onto its surface by the feeding mechanism. In such a circumstance misfeed detector 43 identifies a misaligned copy sheet or absence of a copy sheet and activates a mechanism which rotates arms 304 and 308 in the counterclockwise direction to carry the transfer drum as well as all ofits collateral elements in the counterclockwise direction. When a misfeed signal is received by solenoid 301, solenoid arm 320 moves to the right or away from the transfer drum, bringing cross bar 302, to the right also. Cross bar 302 forces arms 304 and 308 in the counterclockwise direction against the action of spring 377 thereby moving both arms 304 and 308, and the transfer drum, in the counterclockwise direction about shafts 375 and 350. Spring 377 mechanically biases the transfer drum into contact with or in close proximity to the surface of the photoreceptor 90 so that a copy sheet on the surface of the transfer drum contacts the surface of the photoreceptor during the transfer step. By this withdrawal mechanism, the copy sheet can be withdrawn from the transfer station if misaligned or the transfer drum brought away from the transfer station if no copy sheet is on it so that the toner image cannot be transferred to the surface of the transfer drum. This arrangement prevents toner images from being transferred onto the transfer drum itself or onto the copy sheet in any other manner than in perfect register therewith.

Referring to FIG. 7, shaft 375 is mounted in frame 306 and supports arm 304 so that arm 304 can turn on the shaft when cross bar 302 forces the arm 304 to bring the transfer drum away from the photoreceptor. Shafts 350 and 375 have common centerlines and, as a consequence, as cross bar 302 forces frames 304 and 308 to rotate, the drive means for the transfer drum, including gears 351 and 353, moves in tact thereby maintaining the drive relationship. In this manner the position of any point on the transfer drum surface relative to any point on the photoreceptor surface is preserved even though the transfer drum is withdrawn from the photoreceptor because of a misfeeding of the copy sheet. Gear 351 is driven directly off the main shaft of the machine by a belt (not shown) through pulley 352.

The arm 304 can be rotated manually by engaging arm 380 with latch 381. Arm 380 can be rotated in the counterclockwise direction (refer to FIG. 9) by causing shaft 382 to rotate in the counterclockwise direction. Shaft 382 turns counterclockwise by pivoting arm 383 counterclockwise. Arm 383 is supported on brackets 384 and rotates about an axis coincident with shaft 382. As arm 383 pivots in the counterclockwise direction, connecting bars 385, which connect arm 383 and cross bar 302, are forced away from the transfer drum bringing arm 304 in the counterclockwise direction. The combination of the movement of arms 304 and 380 position arm 380 so as to engage the latch 381. Once the latch and arm are in this position, the transfer drum cannot return toward the photoreceptor until latch 381 is released manually. This manual latch feature is especially desirable when maintenance is to be carried out on the transfer drum.

As mentioned above in conjunction with FIG. I, the photoreceptor makes a plurality of revolutions in order to complete a copying cycle and a toner image of one color is transferred to the sheet during each revolution. The copy sheet is fed onto the surface of the transfer drum while the drum rotates at the same speed as the photoreceptor. In order to register and grip the leading edge of the copy sheet with the register stops and gripper fingers as the transfer drum rotates, the copy sheet must be fed at a faster rate of speed than the speed at which the surface of the transfer drum is moving. Referring to FIG. I register rollers 74 feed the leading edge of the copy sheet onto the surface of the transfer drum 42 at a speed which is slightly faster than the speed at which the drum surface is moving and at a time when the register stops and gripper fingers are passing through the 6 oclock position of the transfer drum. After the leading edge of the copy sheet is registered on the register stops, the gripper fingers grip the copy sheet and the copy sheet is carried through the transfer station 40 for three revolutions. During the time that register stops and gripper fingers pass through the 6 oclock position of the transfer drum until just after gripper fingers grip the copy sheet, cam 106 (refer to FIG. 2) assumes its operative position in which it is engaged by the follower 107. After the gripper fingers have secured the copy sheet to the transfer drum surface, cam 201 is immediately moved to its inoperative position. Then, the transfer drum carries the copy sheet through three revolutions during which yellow, cyan and magenta toner images are transferred to the copy sheet. Of course, the cam 201 could be maintained in its operative position for each revolution of the transfer drum, or for any number of revolutions of the transfer drum, if it were desired to do so. After the leading edge of the copy passes through the transfer station for the final time, cam 201 is moved back into its operative position and the gripper fingers are operated to release the copy sheet, the register stops are activated to force the copy sheet away from the surface of the transfer drum and the stripper fingers are operated to separate the copy sheet from the drum and direct its leading edge towards the fusing apparatus. The stripper fingers are moved adjacent the surface of the transfer drum when the copy sheet is placed on the transfer drum also, due to the operative position of the cam during this time. However, the stripping fingers do not interfere with copy sheet being fed onto the drum during this time since they are located at approximately the 1 o'clock position of the drum while the leading edge of copy sheet is placed on the drum at approximately the 6 oclock position.

The transfer of toner images from the photoreceptor to the copy sheet takes place at the transfer station, generally designated 40. During the transfer operation, the copy sheet is secured against the outer periphery of the transfer drum in a manner herein described and the sheets image receiving surface brought into synchronous moving contact with the photoreceptor within the transfer region. During transfer, the core of the transfer drum is electrically connected to a suitable DC source of power capable of biasing the core to a predetermined potential. Since the outer surface of the transfer drum is coated with a layer of non-conductive material, little or no current flows between the photoconductor and the transfer roll. As a result, a relatively strong electrostatic force field is created within the transfer region between the two contacting members. By proper selection of the core polarity, the force field is made to electrically transfer the negatively charged toner particles from the photoreceptor to the receiving surface of the copy sheet.

Conventionally, in most xerographic devices, a toner image is transferred from the photoconductive plate surface to a sheet of final support material using a fixed transfer bias somewhere in the range of between +700 and +2,000 volts. However, the efficiency of the system is considerably reduced when a second image is transferred over the first utilizing this original fixed bias setting. It is believed that the decrease in transfer efficiency is caused by an increase in the resistivity of the receiving body due to the introduction of toner material into the system by the transferringof the first image onto the support sheet. Similarly, when a third image is transferred over the first two images, as is the case in most color processes, the loss of transfer efficiency becomes even more pronounced.

Attempts to compensate for the loss in efficiency by placing the fixed bias at a relatively high level, that is, at a level sufficiently high enough to effectively transfer the third toner image, have not proven to be successful. When utilizing the apparatus herein disclosed it has been found that the bias required to attain this result is so high as to actually inhibit the transfer of the first toner image and in some cases even that of the second image. Although the exact phenomena involved is not fully understood at this time, it is believed the combination of a high bias and a relatively low resistivity in the receiving body during the initial transfer steps results in an air breakdown occurring within the transfer region. This causes the polarity of at least some of the toner particles making up the xerographic image to be reversed and these particles, rather than being attracted towards the copy sheet, are repelled back to the photoconductor during the transfer operation.

Regardless of the phenomena involved, it has been found that there exists some optimum biasing potential range within which each successive toner image can be efficiently transferred and that this range is dictated basically by the amount of new toner material added to the receiving copy sheet surface during the previous transfer steps. 4

The apparatus of the present invention overcomes this undesirable decrease in transfer efficiency where multiple images are to be placed on a single support sheet by changing the operating bias level on the transfer drum between each successive transfer operation, The transfer bias placed on the transfer drum is sequentially increased between each image transfer operation in order to compensate for an increase in the systems resistivity due to the addition of new toner material onto the copy sheet. The increase in bias is stepped in a manner such that each image transfer is accomplished within a range whereby the efficiency of all three image transfer steps is achieved under optimum conditions. For example, good transfer was obtained during all three transfer steps when the initial voltage imposed on the transfer drum was +3,000 volts during the first revolution to transfer the yellow toner image than increase to a +3,50O volts during the second revolution to transfer the cyan toner image in Superposition over the yellow image, and finally increase to a +4,000 volts on the final revolution so as to transfer the magenta toner image in superposition over the first two toner images.

The apparatus for stepping up the voltages on the transfer drum can be any suitable apparatus which functions to increase the voltage in each successive revolution of a copy cycle. For example, FIG. illustrates a control apparatus in block diagram form which is suitable for this purpose. During each revolution of the photoreceptor, two electrical signals are generated from the transfer drum to indicate that the transfer step is about to begin. Any suitable device can be used to generate the signals; for instance, two cams can be placed on the shaft driving the photoreceptor which have follower arms which, in turn, close a switch as transfer begins to allow current to pass through the switch. The signal generating devices illustrated in clock form in FIG. 10 are two such cam switches in conjunction with a power source. One switch is a transfer switch which is closed by its cam and follower arrangement just as the leading edge of the copy sheet enters the transfer station to give the go" signal to the control apparatus to apply a voltage to the transfer drum. The'other switch is a voltage control circuit switch which tells the voltage control circuit that another revolution is being made by the photoreceptor.

The voltage control circuit is a device which has three possible output voltages. The exact output voltage of the control circuit during each transfer step is dependent on how many signals have already been received from the voltage control circuit switch during the copying cycle. At the beginning of the copying cycle in a three color system; that is, during the first revolution of the photoreceptor, a first signal enters the voltage control circuit and the resulting output of the control circuit is the lowest voltage of its three possible output voltages. Then, during the second revolution of the photoreceptor another signal is fed to the voltage control circuit which results in the voltage control circuit stepping up its output to a voltage which is higher than that of the first revolution. Similarly, during the third and final revolution of the photoreceptor, another signal is fed to the voltage control circuit which results in the voltage control circuit stepping up its output to the highest voltage of its three possible output voltages. At this time, the copy cycle has been completed and a new copy cycle begins with a subsequent revolution of the photoreceptor. During the fourth revolution, or the first revolution of a new copying cycle, the voltage control circuit again generates the lowest voltage of its three possible output voltages in response to another signal from the voltage control circuit signal. 7

Each output voltage of the voltage control circuit is passed through a relay control before being fed into the DC power supply. The signal generated by the transfer switch is fed to the relay control also. The relay control allows current to pass from the voltage control circuit outputs to the DC power source only when the go signal has been received from the transfer switch. The

DC power supply, in turn, delivers a voltage to the electrical brush which is a multiple of the voltages fed to it. Assuming that the three output voltages of the voltage control circuit are 30, 35, and40 volts, and the DC power source generates a voltage which is 10 times the voltageit receives from the voltage control circuit, the electrical brush will deliver to, the core of the transfer drum 3,000, 3,500 and 4,000 volts in the first, second, and the third revolutions, respectively, of the photoreceptor.

It is intended that appropriate drive means be associated with the color copying system described herein, and such drive means used can be any suitable type. For instance, the main shaft 91 of the machine can be driven by a main machine motor and the various processing stations around the photoreceptor driven therefrom by a suitable gearing arrangement. in addition, appropriate control circuits can be applied throughout the machine in order to assure that it functions as described above.

In addition to the apparatus outlined above, many other modifications and/or additions to this invention will be readily apparent to those skilled in the art upon reading this disclosure, and these are intended to be encompassed within the invention disclosed and claimed herein.

What is claimed is:

1. The method of producing a copy of a color original upon a sheet of final support material including reducing the original color subject matter into a finite number of color components,

xerographically recording each color component as a charged toner image upon an image retaining plate,

electrically attracting each toner image from said plate to a sheet of final support material whereby at least a portion of each successive image is transferred onto said sheet of final support material in registration over a previously transferred image, and

altering the electrical force of attraction for each successive image transfer step to compensate for changes in the electrical characteristics of the image receiving sheetof final support material due to the presence of a previously transferred image thereon so as to produce optimum image transfer during each successive toner image transfer operation.

2. The method of claim 1 including the further step of fixing said toner image to said receiving body upon the completion of the last transfer step.

3. In a xerographic process for reproducing color originals wherein the original is separated into a number of color components and each of the components is then recorded as a series of charged toner images on a moving photosensitive plate, the plate being adapted to move the color component images seriatim through an image transfer zone, the improvement comprising,

arranging an electrically biased transfer member to move through the transfer zone wherein the transfer member electrically communicates with the charged toner images supported on the plate surface so as to attract the images from the plate towards said transfer member,

securing a sheet of final support material to said transfer member to receive the images attracted thereto,

advancing the sheet on said transfer member repeatedly through the transfer zone in registration with each of the color component images supported on the plate surface whereby at least a portion of each successively transferred toner image placed on the support sheet is superimposed over a previously transferred image, and

increasing the bias potential applied to the transfer member for each successive transfer step to compensate for changes in the electrical characteristics of the image receiving support sheet caused by the addition of reviously transferred charged toner images there 0 whereby each color component image is efficiently transferred from said plate to said receiving body to faithfully recreate the original input scene information.

4. The method of claim 3 further including the step of fixing the color component images to the final support sheet upon the completion of the final transfer operation.

5. The method of claim 3 further including the step of fixing the color component toner images to the final support sheet upon the completion of the final transfer operation.

6. The method of creating a full color copy ofa color original upon a sheet of final support material including separating the original color information into primary color components,

recording a first color component as a charged toner image upon an image retaining plate, placing a sheet of final support material in overlying contact with said first color component image,

establishing an electrical field within the contact zone to attract the first color component image from said plate to said sheet of final support material,

removing said sheet from said plate and recording a second color component as a charged toner image on said plate,

replacing the support sheet in overlying contact with the second color component image on said plate whereby at least a portion of said second color component image is superimposed in registration over said first image,

establishing a second electrical field within the contact zone to attract the second color component image in superposition over said first color component image, said second electrical field being of a strength greater than said first electrical field to compensate for changes in the electrical characteristics of the final support material caused by the deposition of the first color component image thereon,

removing the sheet from said plate and repeating the image recording and transfer steps to' produce a full color rendition of the original subject matter on said final support sheet.

7. The method of claim 6 wherein the original subject matter is separated into the primary color components of red, green and blue and images thereof recorded with cyan, magenta and yellow toners.

8. The method of claim 7 wherein the strength of the electrical transfer field is increased uniformly after the completion of each image transfer step. 

2. The method of claim 1 including the further step of fixing said toner image to said receiving body upon the completion of the last transfer step.
 3. In a xerographic process for reproducing color originals wherein the original is separated into a number of color components and each of the components is then recorded as a series of charged toner images on a moving photosensitive plate, the plate being adapted to move the color component images seriatim through an image transfer zone, the improvement comprising, arranging an electrically biased transfer member to move through the transfer zone wherein the transfer member electrically communicates with the charged toner images supported on the plate surface so as to attract the images from the plate towards said transfer member, securing a sheet of final support material to said transfer member to receive the images attracted thereto, advancing the sheet oN said transfer member repeatedly through the transfer zone in registration with each of the color component images supported on the plate surface whereby at least a portion of each successively transferred toner image placed on the support sheet is superimposed over a previously transferred image, and increasing the bias potential applied to the transfer member for each successive transfer step to compensate for changes in the electrical characteristics of the image receiving support sheet caused by the addition of previously transferred charged toner images thereto whereby each color component image is efficiently transferred from said plate to said receiving body to faithfully recreate the original input scene information.
 4. The method of claim 3 further including the step of fixing the color component images to the final support sheet upon the completion of the final transfer operation.
 5. The method of claim 3 further including the step of fixing the color component toner images to the final support sheet upon the completion of the final transfer operation.
 6. The method of creating a full color copy of a color original upon a sheet of final support material including separating the original color information into primary color components, recording a first color component as a charged toner image upon an image retaining plate, placing a sheet of final support material in overlying contact with said first color component image, establishing an electrical field within the contact zone to attract the first color component image from said plate to said sheet of final support material, removing said sheet from said plate and recording a second color component as a charged toner image on said plate, replacing the support sheet in overlying contact with the second color component image on said plate whereby at least a portion of said second color component image is superimposed in registration over said first image, establishing a second electrical field within the contact zone to attract the second color component image in superposition over said first color component image, said second electrical field being of a strength greater than said first electrical field to compensate for changes in the electrical characteristics of the final support material caused by the deposition of the first color component image thereon, removing the sheet from said plate and repeating the image recording and transfer steps to produce a full color rendition of the original subject matter on said final support sheet.
 7. The method of claim 6 wherein the original subject matter is separated into the primary color components of red, green and blue and images thereof recorded with cyan, magenta and yellow toners.
 8. The method of claim 7 wherein the strength of the electrical transfer field is increased uniformly after the completion of each image transfer step. 